This invention relates to electrostatography, and in particular, to an apparatus for cleaning a corona discharge device.
In the practice of xerography as described in U.S. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.
The latent electrostatic image may then be developed by contacting it with a finely divided electrostatically attractable material, such as a resinous powder. The powder is held in the image areas by the electrostatic fields on the layer. Where the field is greatest, the greatest amount of material is deposited, where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably affixed to thereby form a permanent print.
In the electrophotographic reproducing art, it is necessary to deposit a uniform layer of electrostatic charges on the surface of a photoreceptor such that the electrostatic charges may be selectively dissipated in accordance with modulated radiation imaged thereon to form an electrostatic latent image of an original document. The prior art has suggested various techniques and devices for applying a uniform electrostatic charge on the surface of a photoreceptor. One technique utilizes a corona discharge device of the type described in U.S. Pat. No. 2,777,957 which issued to L. E. Walkup. A particular type of corona discharge device that has been readily included in conventional electrophotographic reproducing apparatus is described in detail in U.S. Pat. No. 2,836,725 which issued to R. G. Vyverberg and assigned to Xerox Corporation. Such a device is usually comprised of a corona discharge electrode, such as a corona wire, surrounded by a conductive shield. The corona discharge electrode is adapted to be supplied with a DC voltage of sufficient magnitude to create a corona current flow from the electrode to the surface of a photoreceptor in spaced relationship therefrom. The geometry of a typical corona discharge device admits of various configurations as disclosed in U.S. Pat. No. 2,879,295 which issued to L. E. Walkup and assigned to Xerox Corporation.
Although the corona discharge device is advantageously utilized to deposit a uniform layer of electrostatic charge on the surface of a photoreceptor, various other applications thereof have been adopted. Typical of these applications are electrostatic transfer of a developed image to an image receiving member, removal of background toner particles from a developed electrostatic latent image and pre-cleaning of a photoreceptor by neutralizing the charge on toner particles adhering to the surface of the photoreceptor after transfer of the developed image to an image receiving member. An attendant disadvantage of corona discharge devices developed by the prior art for use in electrophotographic reproducing devices is the accumulation of dust particles and toner particles on and about the interior of the corona discharge device to such an extent that the corona current generated thereby substantially decreases as the density of particle accumulation increases. Accordingly, self-cleaning corona discharge devices have been developed which employ corona winds inherently generated by a corona discharge device as a means to clean the corona discharge electrode and the interior walls of the surrounding shield. Detailed descriptions of such devices may be found in U.S. Pat. No. 3,324,291 which issued to F. W. Hudson on June 6, 1967 and assigned to Xerox Corporation and in U.S. Pat. No. 3,471,695 which issued to F. W. Hudson et al on Oct. 7, 1969 and assigned to Xerox Corporation.
An alternative embodiment of a self-cleaning corona discharge device comprises an AC corona discharge device wherein the corona discharge electrode thereof is supplied with a corona generating AC voltage. As is understood by those of ordinary skill in the art, the creation of a corona current is predominantly determined by the potential difference between the corona discharge electrode and the partially surrounding shield. Accordingly, if the shield is comprised of a metal shield supplied with a reference potential, such as ground potential, a positive corona current will be generated when the difference between the AC voltage applied to the corona discharge electrode and the reference potential supplied to the metal shield exceeds the positive corona threshold voltage; and a negative corona current will be generated when the difference between the AC voltage applied to the corona discharge electrode and the reference potential supplied to the metal shield exceeds the negative corona threshold. In theory then, a maximum AC corona current is generated for the embodiment of an AC corona discharge device including a grounded metal shield. Unfortunately, the grounded shield AC corotron suffers from the disadvantage that toner or dirt particles which accumulate on the inner walls of the metal shield and are not effectively removed therefrom produce deleterious effects. More specifically, these particles are comprised of dielectric material which store the charged ions communicated thereto from the corona discharge electrode during the discharge. As charge build-up on these particles occurs, a voltage is induced on the contaminated inner wall of the surrounding shield resulting in a non-uniform potential difference between the corona discharge electrode and the shield. Hence, the AC corona current is subject to variations due to the discontinuities in the aforementioned potential difference.
A suggested improvement over the grounded metal shield AC corona discharge device is comprised of an AC corona discharge device having an insulating or dielectric shield constructed of a plastic material. The accumulation of dielectric particles on the inner surface of the dielectric shield has no appreciable effect upon the AC corona current. This advantageous characteristic is however, achieved at the loss of magnitude of the AC corona current.